It all started with a pudding: A brief history of the Atom


How does this work? Image Credit: Chetvorno. [Public Domain]. Via: Wikimedia Commons

From philosophers in ancient Greece to modern day physicists and chemists, people have always sought to understand matter and to  answer the question: What is stuff made of? After much guesswork and speculation throughout the years, an experiment was finally conducted. In 1897, J.J. Thomson discovered the electron via a cathode ray experiment.

Thomson’s Pudding Hypothesis:


Thomson’s Plum Pudding Model. Image Credit: Kurzon. [CC BY-SA 3.0]. Via: Wikimedia Commons

The discovery of the electron led Thomson to propose a model for the atom known  as the Plum Pudding Model. Thomson knew that atoms were neutral in charge with negatively charged electrons. As such, there had to be an equal source of positive charge within the atom to balance the negative charge of the electrons.

Thomson hypothesised that the atom consisted of electrons within a ‘soup’ of diffuse positive charge. Thus, if you fire a stream of positively charged particles through the atom, you would expect it to pass straight through as most of the atom would simply be empty space with the charges well spaced out. This hypothesis was however found to be false by Ernest Rutherford in 1911.

Rutherford’s Gold Foil Experiment:


Rutherford’s Gold Foil Experiment. Image Credit: Kurzon. [CC BY-SA 3.0]. Via: Wikimedia Commons

In 1911, Ernest Rutherford decided to test out Thomson’s model by conducting an experiment known as the “Gold Foil Experiment”. When positively charged particles were fired at a thin piece of gold foil, most of the particles passed straight through. A few particles however were deflected sharply out of the atom with some being reflected directly backwards. Thus, there had to be something within the atom causing the deflection.

Rutherford then proposed a new model for the atom based on his findings: a small concentrated core of positive charge with negatively charged electrons orbiting the nucleus, just as planets orbit the sun. Scientists around the world were excited and intrigued. Finally, there was a model of the atom that could be verified experimentally.

Rutherford and a fellow scientist Chadwick later described the core of the nucleus after many experiments as having two different particles:

  • Positively charged Protons 
  • Uncharged neutral particles known as Neutrons

This model of the atom was all well and good but the quest was far from over. Scientist’s now wanted to find out more about the electrons. What were their properties?

Quantum Mechanics:


 An accurate model for Hydrogen! Image Credit: PoorLeno. [Public Domain]. Via: Wikimedia Commons

Previous attempts by Niels Bohr, a famous Danish physicist, to explain atomic structure by likening them to planetary orbits were unsatisfactory. While Bohr’s model worked for the hydrogen atom, it failed to account for electronic energies in the other elements. By the mid 1930s however, a revolutionary theory known as Quantum Mechanics had been developed which was able to accurately describe the atomic structure of the elements in agreement with experimental data.

The principle underlying most of quantum mechanics is that particles such as electrons can behave like waves as well. By treating the electron as having wave-like properties, physicists were able to describe the atomic structure of the elements in a way that agreed with experimental data. Instead of orbiting the nucleus, electrons were described using ‘Orbitals'(see picture above).

At the forefront of the theory’s development were four prominent physicists: Albert Einstein, Erwin Schrodinger, Louis De Broglie and Werner Heisenberg. Together, they laid the foundations for modern quantum theory.

Quantum Mechanics is an ever-expanding and fascinating field. To find out more about what has been mentioned check out the following links!:

Einstein’s Nobel Prize 1921 – Einstein showed how Light could be treated as particle and as a wave at the same time

Orbitals – What are they?

Schrodinger’s Work and his Equation 

The DeBroglie Wavelength

Heisenberg’s Uncertaity Principle